corner  

The corner eye shape in this diagram is known as bent four in the corner. If Black has one or zero outside liberties, as in this diagram, then White can create a ko for the life of the black group by play at the marked point. This is a favorable ko for white, because black has to throw in at the 1-1 point to start the ko, allowing white to take first.

More important than the status of this specific shape, however, is what happens if a player can almost fill a group with a bent four. In such cases, the player can leave the group until the end of the game before starting the ko. Since the attacker takes the ko first, this usually means that the group cannot be saved, and that the players need not even play the ko. This is the situation known as "bent four in the corner is dead". Because the group is not dead outright, but only through ko, the status of the group may depend on the rule set in use, and the possible availability of unremovable ko threats. It is a famous example of a situation that has posed a challenge to many rules writers.